Similar Star Formation Histories Research Articles

Chemodynamical Nature of the Anticenter Stream and Monoceros Ring

In the epoch of deep photometric surveys, a large number of substructures—e.g., overdensities and streams—have been identified. With the help of astrometry and spectroscopy, the community has revealed a complex picture of our Milky Way (MW) after investigating their origins. The off-plane substructures the Anticenter Stream (ACS) and Monoceros Ring (MNC), once considered as dissolving dwarf galaxies, were later found to share similar kinematics and metallicity with the Galactic outer thin disk. In this work, we aim to chemically tag ACS and MNC with high-accuracy abundances from the APOGEE survey. By extrapolating chemical abundance trends in the outer thin-disk region (10 < R GC < 18 kpc, 0 < ∣Z GC∣ < 3 kpc), we found that ACS and MNC stars show consistent chemical abundances as the extrapolating values for 12 elements, including C, N, O, Mg, Al, Si, K, Ca, Cr, Mn, Co, and Ni. The similar chemical patterns indicate that ACS and MNC have a similar star formation history as the MW outer thin disk, while we also excluded their dwarf galaxy association, as they are distinctive in multiple chemical spaces. The ages of ACS and MNC stars are consistent with the time of the first Sagittarius dSph passage, indicating their possible connection.

The MOSDEF survey: a new view of a remarkable z = 1.89 merger

ABSTRACT We present a detailed study of a galaxy merger taking place at z = 1.89 in the GOODS-S field. Here, we analyse Keck/MOSFIRE spectroscopic observations from the MOSFIRE Deep Evolution Field (MOSDEF) survey along with multiwavelength photometry assembled by the 3D-HST survey. The combined data set is modelled to infer the past star formation histories (SFHs) of both merging galaxies. They are found to be massive, with log10(M*/M⊙) &amp;gt; 11, with a close mass ratio satisfying the typical major-merger definition. Additionally, in the context of delayed-τ models, GOODS-S 43114, and GOODS-S 43683 have similar SFHs and low star formation rates (log10(SFR(SED)/${\rm M}_{\odot }\,\rm {yr}^{-1}$) &amp;lt; 1.0) compared to their past averages. The best-fitting model SEDs show elevated H δA values for both galaxies, indicating that their stellar spectra are dominated by A-type stars, and that star formation peaked ∼0.5−1 Gyr ago and has recently declined. Additionally, based on SED fitting both merging galaxies turned on and shut off star formation within a few hundred Myr of each other, suggesting that their bursts of star formation may be linked. Combining the SFHs and H δA results with recent galaxy merger simulations, we infer that these galaxies have recently completed their first pericentric passage and are moving apart. Finally, the relatively low second velocity moment of GOODS-S 43114, given its stellar mass suggests a disc-like structure. However, including the geometry of the galaxy in the modelling does not completely resolve the discrepancy between the dynamical and stellar masses. Future work is needed to resolve this inconsistency in mass.

Evolution of the Ultraviolet Upturn at 0.3 &lt; z &lt; 1: Exploring Helium-rich Stellar Populations

Abstract We measure the near-UV (rest-frame ∼2400 Å) to optical color for early-type galaxies in 12 clusters at 0.3 &lt; z &lt; 1.0. We show that this is a suitable proxy for the more common far-ultraviolet bandpass used to measure the ultraviolet upturn and find that the upturn is detected to z = 0.6 in these data, in agreement with previous work. We find evidence that the strength of the upturn starts to wane beyond this redshift and largely disappears at z = 1. Our data are most consistent with models where early-type galaxies contain minority stellar populations with non-cosmological helium abundances, up to around 46%, formed at z ≥ 3, resembling globular clusters with multiple stellar populations in our Galaxy. This suggests that elliptical galaxies and globular clusters share similar chemical evolution and star formation histories. The vast majority of the stellar mass in these galaxies also must have been in place at z &gt; 3.

LLAMA: Stellar populations in the nuclei of ultra-hard X-ray-selected AGN and matched inactive galaxies

The relation between nuclear (≲50 pc) star formation and nuclear galactic activity is still elusive; theoretical models predict a link between the two, but it is unclear whether active galactic nuclei (AGNs) should appear at the same time, before, or after nuclear star formation activity. We present a study of this relation in a complete, volume-limited sample of nine of the most luminous (log L14 − 195 keV &gt; 1042.5 erg s−1) local AGNs (the LLAMA sample), including a sample of 18 inactive control galaxies (six star-forming; 12 passive) that are matched by Hubble type, stellar mass (9.5 ≲ log M⋆/M⊙ ≲ 10.5), inclination, and distance. This allows us to calibrate our methods on the control sample and perform a differential analysis between the AGN and control samples. We performed stellar population synthesis on VLT/X-shooter spectra in an aperture corresponding to a physical radius of ≈150 pc. We find young (≲30 Myr) stellar populations in seven out of nine AGNs and in four out of six star-forming control galaxies. In the non-star-forming control population, in contrast, only two out of 12 galaxies show such a population. We further show that these young populations are not indicative of ongoing star formation, providing evidence for models that see AGN activity as a consequence of nuclear star formation. Based on the similar nuclear star formation histories of AGNs and star-forming control galaxies, we speculate that the latter may turn into the former for some fraction of their time. Under this assumption, and making use of the volume completeness of our sample, we infer that the AGN phase lasts for about 5% of the nuclear starburst phase.

Linking globular cluster formation at low and high redshift through the age–metallicity relation in E-MOSAICS

ABSTRACT We set out to compare the age–metallicity relation (AMR) of massive clusters from Magellanic Cloud mass galaxies in the E-MOSAICS suite of numerical cosmological simulations with an amalgamation of observational data of massive clusters in the Large and Small Magellanic Clouds (LMC/SMC). We aim to test if: (i) star cluster formation proceeds according to universal physical processes, suggestive of a common formation mechanism for young-massive clusters (YMCs), intermediate-age clusters (IACs), and ancient globular clusters (GCs); (ii) massive clusters of all ages trace a continuous AMR; and (iii) the AMRs of smaller mass galaxies show a shallower relation when compared to more massive galaxies. Our results show that, within the uncertainties, the predicted AMRs of L/SMC-mass galaxies with similar star formation histories to the L/SMC follow the same relation as observations. We also find that the metallicity at which the AMR saturates increases with galaxy mass, which is also found for the field star AMRs. This suggests that relatively low-metallicity clusters can still form in dwarfs galaxies. Given our results, we suggest that ancient GCs share their formation mechanism with IACs and YMCs, in which GCs are the result of a universal process of star cluster formation during the early episodes of star formation in their host galaxies.

Elemental Abundances in M31: Properties of the Inner Stellar Halo*

Abstract We present measurements of [Fe/H] and [α/Fe] for 128 individual red giant branch stars (RGB) in the stellar halo of M31, including its Giant Stellar Stream (GSS), obtained using spectral synthesis of low- and medium-resolution Keck/DEIMOS spectroscopy ( and 6000, respectively). We observed four fields in M31's stellar halo (at projected radii of 9, 18, 23, and 31 kpc), as well as two fields in the GSS (at 33 kpc). In combination with existing literature measurements, we have increased the sample size of [Fe/H] and [α/Fe] measurements from 101 to a total of 229 individual M31 RGB stars. From this sample, we investigate the chemical abundance properties of M31's inner halo, finding and . Between 8 and 34 kpc, the inner halo has a steep [Fe/H] gradient (−0.025 ± 0.002 dex kpc−1) and negligible [α/Fe] gradient, where substructure in the inner halo is systematically more metal-rich than the smooth component of the halo at a given projected distance. Although the chemical abundances of the inner stellar halo are largely inconsistent with that of present-day dwarf spheroidal (dSph) satellite galaxies of M31, we identified 22 RGB stars kinematically associated with the smooth component of the stellar halo that have chemical abundance patterns similar to M31 dSphs. We discuss formation scenarios for M31's halo, concluding that these dSph-like stars may have been accreted from galaxies of similar stellar mass and star formation history, or of higher stellar mass and similar star formation efficiency.

A Closer Look at Bursty Star Formation with LHα and LUV Distributions

Abstract We investigate the bursty star formation histories (SFHs) of dwarf galaxies using the distribution of log( ) of 185 local galaxies. We expand on the work of Weisz et al. to consider a wider range of SFHs and stellar metallicities, and show that there are large degeneracies in a periodic, top-hat burst model. We argue that all galaxies of a given mass have similar SFHs and we can therefore include the L Hα distributions (subtracting the median trend with stellar mass, referred to as ) in our analyses. traces the amplitude of the bursts, and log( ) is a function of the timescale, amplitude, and shape of the bursts. We examine the two-dimensional distribution of these two indicators to constrain the SFHs. We use exponentially rising/falling bursts to determine timescales (e-folding time, τ). We find that galaxies below 107.5 M ⊙ undergo large (maximum amplitudes of ∼100) and rapid (τ &lt; 30 Myr) bursts, while galaxies above 108.5 M ⊙ experience smaller (maximum amplitudes ∼10), slower (τ ≳ 300 Myr) bursts. We compare with the FIRE-2 hydrodynamical simulations and find that the burst amplitudes agree with observations, but they are too rapid in intermediate-mass galaxies ( M ⊙). Finally, we confirm that stochastic sampling of the stellar mass function cannot reproduce the observed distributions unless the standard assumptions of cluster and stellar mass functions are changed. With the next generation of telescopes, measurements of L UV and L Hα will become available for dwarf galaxies at high redshift, enabling similar analyses of galaxies in the early universe.

Identifying Sagittarius Stream Stars by Their APOGEE Chemical Abundance Signatures

Abstract The SDSS-IV Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey provides precise chemical abundances of 18 chemical elements for ∼176,000 red giant stars distributed over much of the Milky Way Galaxy (MW), and includes observations of the core of the Sagittarius dwarf spheroidal galaxy (Sgr). The APOGEE chemical abundance patterns of Sgr have revealed that it is chemically distinct from the MW in most chemical elements. We employ a k-means clustering algorithm to six-dimensional chemical space defined by [(C+N)/Fe], [O/Fe], [Mg/Fe], [Al/Fe], [Mn/Fe], and [Ni/Fe] to identify 62 MW stars in the APOGEE sample that have Sgr-like chemical abundances. Of the 62 stars, 35 have Gaia kinematics and positions consistent with those predicted by N-body simulations of the Sgr stream, and are likely stars that have been stripped from Sgr during the last two pericenter passages (&lt;2 Gyr ago). Another 20 of the 62 stars exhibit chemical abundances indistinguishable from the Sgr stream stars, but are on highly eccentric orbits with median r apo ∼ 25 kpc. These stars are likely the “accreted” halo population thought to be the result of a separate merger with the MW 8–11 Gyr ago. We also find one hypervelocity star candidate. We conclude that Sgr was enriched to [Fe/H] ∼ −0.2 before its most recent pericenter passage. If the “accreted halo” population is from one major accretion event, then this progenitor galaxy was enriched to at least [Fe/H] ∼ −0.6, and had a similar star formation history to Sgr before merging.

Nuclei of dwarf spheroidal galaxies KKs 3 and ESO 269−66 and their counterparts in our Galaxy

We present the analysis of medium-resolution spectra obtained at the Southern African Large Telescope (SALT) for nuclear globular clusters (GCs) in two dwarf spheroidal galaxies (dSphs). The galaxies have similar star formation histories, but they are situated in completely different environments. ESO269-66 is a close neighbour of the giant S0 NGC5128. KKs3 is one of the few truly isolated dSphs within 10 Mpc. We estimate the helium abundance $Y=0.3$, $\rm age=12.6\pm1$ Gyr, $[Fe/H]=-1.5,-1.55\pm0.2$ dex, and abundances of C, N, Mg, Ca, Ti, and Cr for the nuclei of ESO269-66 and KKs3. Our surface photometry results using HST images yield the half-light radius of the cluster in KKs3, $\rm r_h=4.8\pm0.2$ pc. We demonstrate the similarities of medium-resolution spectra, ages, chemical compositions, and structure for GCs in ESO269-66 and KKs3 and for several massive Galactic GCs with $[Fe/H]\sim-1.6$ dex. All Galactic GCs posses Extended Blue Horizontal Branches and multiple stellar populations. Five of the selected Galactic objects are iron-complex GCs. Our results indicate that the sample GCs observed now in different environments had similar conditions of their formation $\sim$1 Gyr after the Big Bang.

THE IMPACT OF STELLAR FEEDBACK ON THE STRUCTURE, SIZE, AND MORPHOLOGY OF GALAXIES IN MILKY-WAY-SIZED DARK MATTER HALOS

ABSTRACT We use cosmological zoom-in simulations of galaxy formation in a Milky-Way-sized halo started from identical initial conditions to investigate the evolution of galaxy sizes, baryon fractions, morphologies, and angular momenta in runs with different parameters of the star formation–feedback cycle. Our fiducial model with a high local star formation efficiency, which results in efficient feedback, produces a realistic late-type galaxy that matches the evolution of basic properties of late-type galaxies: stellar mass, disk size, morphology dominated by a kinematically cold disk, stellar and gas surface density profiles, and specific angular momentum. We argue that feedback’s role in this success is twofold: (1) removal of low angular momentum gas, and (2) maintaining a low disk-to-halo mass fraction, which suppresses disk instabilities that lead to angular momentum redistribution and a central concentration of baryons. However, our model with a low local star formation efficiency, but large energy input per supernova, chosen to produce a galaxy with a similar star formation history as our fiducial model, leads to a highly irregular galaxy with no kinematically cold component, overly extended stellar distribution, and low angular momentum. This indicates that only when feedback is allowed to become vigorous via locally efficient star formation in dense cold gas do resulting galaxy sizes, gas/stellar surface density profiles, and stellar disk angular momenta agree with observed z = 0 galaxies.

Mass–metallicity relation for AKARI-FMOS infrared luminous galaxies at z ~ 0.9

AbstractWe study the mass–metallicity relation and fundamental relation (FMR) for infrared bright galaxies (IR galaxies) at z ~ 0.9 discovered by AKARI NEP-Deep survey. The main result of this work is that metallicity of IR galaxies surprisingly match optical selected galaxies at a given mass even their star formation rates are different, which may imply that optical and IR selected galaxies follow similar star formation histories, and the starbursts in the IR galaxies do not give a strong impact in changing metallicity because of the short duration time.

A STATISTICAL STUDY OF H i GAS IN NEARBY NARROW-LINE AGN-HOSTING GALAXIES

As a quenching mechanism, AGN feedback could suppress on-going star formation in their host galaxies. On the basis of a sample of galaxies selected from ALFALFA HI survey, the dependence of their HI mass M[HI], stellar mass M[*] & HI-to-stellar mass ratio M[HI]/M[*] on various tracers of AGN activity are presented and analyzed in this paper. Almost all the AGN-hostings in this sample are gas-rich galaxies, and there is no any evidence to be shown to indicate that the AGN activity could increase/decrease either M[HI] or M[HI]/M[*]. The cold neutral gas can not be fixed positions accurately just based on available HI data due to the large beam size of ALFALFA survey. In addition, even though AGN-hostings are more easily detected by HI survey compared with absorption line galaxies, these two types of galaxies show similar star formation history. If an AGN-hosting would ultimately evolve into an old red galaxy with few cold gas, then when and how the gas has been exhausted have to be solved by future hypotheses and observations.

TRACING CHEMICAL EVOLUTION OVER THE EXTENT OF THE MILKY WAY'S DISK WITH APOGEE RED CLUMP STARS

We employ the first two years of data from the near-infrared, high-resolution SDSS-III/APOGEE spectroscopic survey to investigate the distribution of metallicity and alpha-element abundances of stars over a large part of the Milky Way disk. Using a sample of ~10,000 kinematically-unbiased red-clump stars with ~5% distance accuracy as tracers, the [alpha/Fe] vs. [Fe/H] distribution of this sample exhibits a bimodality in [alpha/Fe] at intermediate metallicities, -0.9<[Fe/H]<-0.2, but at higher metallicities ([Fe/H]=+0.2) the two sequences smoothly merge. We investigate the effects of the APOGEE selection function and volume filling fraction and find that these have little qualitative impact on the alpha-element abundance patterns. The described abundance pattern is found throughout the range 5<R<11 kpc and 0<|Z|<2 kpc across the Galaxy. The [alpha/Fe] trend of the high-alpha sequence is surprisingly constant throughout the Galaxy, with little variation from region to region (~10%). Using simple galactic chemical evolution models we derive an average star formation efficiency (SFE) in the high-alpha sequence of ~4.5E-10 1/yr, which is quite close to the nearly-constant value found in molecular-gas-dominated regions of nearby spirals. This result suggests that the early evolution of the Milky Way disk was characterized by stars that shared a similar star formation history and were formed in a well-mixed, turbulent, and molecular-dominated ISM with a gas consumption timescale (1/SFE) of ~2 Gyr. Finally, while the two alpha-element sequences in the inner Galaxy can be explained by a single chemical evolutionary track this cannot hold in the outer Galaxy, requiring instead a mix of two or more populations with distinct enrichment histories.

The star formation history of luminous red galaxies hosting Mg ii absorbers★

We present a spectroscopic sample of z \approx 0.5 luminous red galaxies (LRGs) that are located within physical projected distances \rho \leq 350 kpc/h of a QSO sightline. Of the 37 LRGs in our sample, eight have associated MgII absorbers with rest-frame equivalent width Wr(2796)>0.3A and velocity separation |dv| \leq 350 km/s and 29 do not have associated MgII absorbers to a 2-sigma limit of Wr(2796)=0.3A. We perform a stellar population synthesis analysis using stacked spectra of the MgII absorbing and non-absorbing LRG subsamples. We find that LRGs with or without associated MgII absorbers share similar star formation histories and are best described by old stellar population models (\geq 1 Gyr). Younger stellar populations (\leq 1 Gyr) fail to reproduce their spectra. These findings are consistent with the lack of [OII] emission features in the LRG spectra. The primarily old stellar populations in the LRGs indicate that starburst driven outflows are unlikely to explain the observed MgII absorbers at large distances from the LRGs. In addition, the spectroscopic LRG sample allows us to derive a sensitive constraint for the cool gas covering fraction of <\kappa>=14 +/- 6% in the LRG halos for absorbers of Wr(2796)>0.3A. Finally, we speculate on the origin of the observed MgII absorbers around the LRGs.

Spatially Resolved Stellar Populations in Dwarf and Giant Galaxies

The dwarf elliptical galaxies (dEs) are more luminous than dwarf spheroidals and fainter than “normal” ellipticals. In this paper I investigate the interconnection between these three families of early-type galaxies by studying their stellar populations properties. Some dwarf ellipticals have strong metallicity and age gradients, up to − 0.5 dex and 0.2 dex per decade of radius, respectively. The dispersion of the gradients is similar to the one observed in the dwarf spheroidals and normal ellipticals and there is no gradient-luminosity relation. The star formation history of dEs is characterised by a mass-dominant old population, generally prolonged to intermediate ages. This pattern is apparently independent of the environment (scarce groups or dense clusters), as confirmed by the observations of the Local Group dwarf spheroidals. At higher masses, the S0s have a similar star formation history, while the tail of late star formation is rarer and shorter in elliptical galaxies ( M B ~ −19). I conclude that the transition between different classes is smooth, without any abrupt changes in their stellar population properties.

THE PROPERTIES OF QUASAR HOSTS AT THE PEAK OF THE QUASAR ACTIVITY

We present near-infrared imaging obtained with ESO VLT/ISAAC of a sample of 16 low luminosity radio-quiet quasars at the epoch around the peak of the quasar activity (2 < z < 3), aimed at investigating their host galaxies. For 11 quasars, we are able to detect the host galaxies and derive their properties, while for the other five quasars, upper limits to the host luminosity are estimated. The luminosities of the host galaxies of radio-quiet quasars at high redshift are in the range of those of massive inactive elliptical galaxies. This work complements our previous systematic study of quasar hosts aimed to trace the cosmological luminosity evolution of the host galaxies up to z ~2 and extends our pilot study of a few luminous quasars at z > 2. The luminosity trend with cosmic epoch resembles that observed for massive inactive galaxies, suggesting a similar star formation history. In particular, both quasar host galaxies and massive inactive galaxies appear mostly assembled already at the peak age of the quasar activity. This result is of key importance for testing the models of joint formation and evolution of galaxies and their active nuclei.

Attenuation laws for galaxies with different star formation histories, based on the SDSS images

Attenuation curves (laws) in the optical part of the spectrum (from about 3500 A to 9000 A) are determined for galaxies with different star formation histories (SFHs) using images in five photometrical bands from the Sloan Digital Sky Survey (SDSS). Owing to large surveys like SDSS, it is possible to define several subgroups of galaxies with similar SFHs using two spectral indices that trace SFH. Attenuation curves are analyzed in terms of SFH and compared to the curves that are often used for dust correction of the observed data. .

Abundances and Kinematics for a Sample of LMC Clusters

AbstractWe present results from a project aimed at better understanding the kinematics and metallicities of populous clusters in the LMC. In an effort to update previous [Fe/H] determinations, we have utilized FORS2 on the VLT to obtain infrared spectra for more than 200 stars in 28 LMC clusters. The absorption lines of the calcium II triplet were then used to calculate radial velocities and [Fe/H] for a sample of clusters spanning a large range of ages (~ 1-13 Gyr) and metallicities (−0.3 &gt;[Fe/H]&gt; − 2.0). We determine mean cluster velocities to typically 1.6 km s−1 and mean metallicities to 0.04 dex (random error). For eight of these clusters, we report the first spectroscopically determined metallicities based on individual cluster stars, and six of these eight have no published radial velocity measurements. Combining our data with archival HST/WFPC2 photometry, we find the newly measured cluster, NGC 1718, is one of the most metal-poor ([Fe/H] ~ −0.80), intermediate age (~ 2 Gyr) inner disk clusters in the LMC. Similar to what was found by previous authors, this cluster sample has motions consistent with that of a single rotating disk system, with no indication that the newly reported clusters exhibit halo kinematics. Additionally, our findings confirm previous results which show that the LMC lacks the metallicity gradient typically seen in non-barred spiral galaxies, suggesting that the bar is driving the mixing of stellar populations in the LMC. However, in contrast to previous work, we find that the higher metallicity clusters (≥−1.0 dex) in our sample show a very tight distribution (mean [Fe/H] = −0.48, σ = 0.09), with no tail toward solar metallicities. The cluster distribution is similar to what has been found for red giant stars in the bar, which indicates that the bar and the intermediate age clusters have similar star formation histories. This is in good agreement with recent theoretical models that suggest the bar and intermediate age clusters formed as a result of a close encounter with the SMC ~ 4 Gyr ago.

The IMF and star formation history of the stellar clusters in the Vela D cloud

We present the results of a Near-Infrared deep photometric survey of a sample of six embedded star clusters in the Vela-D molecular cloud, all associated with luminous (~10^3 Lsun) IRAS sources. The clusters are unlikely to be older than a few 10^6 yrs, since all are still associated with molecular gas. We employed the fact that all clusters lie at the same distance and were observed with the same instrumental setting to derive their properties in a consistent way, being affected by the same instrumental and observational biases. We extracted the clusters' K Luminosity Functions (KLF) and developed a simple method to correct them for extinction, based on colour-magnitude diagrams. The reliability of the method has been tested by constructing synthetic clusters from theoretical tracks for pre-main sequence stars and a standard Initial Mass Function (IMF). The clusters' IMFs have been derived from the dereddened KLFs by adopting a set of pre-main sequence evolutionary tracks and assuming coeval star formation. All clusters are small (~100 members) and compact (radius \~0.1-0.2 pc); their most massive stars are intermediate-mass (~2-10 Msun) ones. The dereddened KLFs are likely to arise from the same distribution, suggesting that the selected clusters have quite similar IMFs and star formation histories. The IMFs are consistent with those derived for field stars and clusters. Adding them together we found that the ``global'' IMF appears steeper at the high-mass end and exhibits a drop-off at ~10 Msun. In fact, a standard IMF would predict a star with M>22.5 Msun within one of the clusters, which is not found. Hence, either high-mass stars need larger clusters to be formed, or the IMF of the single clusters is steeper at the high-mass end because of the physical conditions in the parental gas.

Spectrophotometry of Sextans A and B: Chemical Abundances of HiiRegions and Planetary Nebulae

We present the results of high-quality long-slit spectroscopy of planetary nebulae (PNe) and H II regions in the two dwarf irregular (dIrr) galaxies Sextans A and B, which belong to a small group of galaxies just outside the Local Group. The observations were obtained with the New Technology Telescope ESO Multi-Mode Instrument. In Sextans A we obtained the element abundances in its only known PN and in three H II regions with the classical Te method. The oxygen abundances in these three H II regions of Sextans A are all consistent within the individual rms uncertainties, with an average 12 + log(O/H) = 7.54 ± 0.06. The oxygen abundance of the PN in Sextans A is, however, significantly higher: 12 + log(O/H) = 8.02 ± 0.05. This PN is even more enriched in nitrogen and helium, suggesting a classification as a PN of type I. The PN abundances of S and Ar, which are presumably unaffected by nucleosynthesis in the progenitor star, are well below those in the H II regions, indicating lower metallicity at the epoch of the PN progenitor formation (~1.5 Gyr ago, according to our estimates based on the PN parameters). In Sextans B we obtained spectra of one PN and six H II regions. Element abundances with the Te method could be derived for the PN and three of the H II regions. For two of these H II regions, which have a separation of only ~70 pc in projection, the oxygen abundances do not differ within the rms uncertainties, with a mean of 12 + log(O/H) = 7.53 ± 0.05. The third H II region, which is about 0.6 kpc northeast of the first two, is twice as metal-rich, with 12 + log(O/H) = 7.84 ± 0.05. This suggests considerable inhomogeneity in the present-day metallicity distribution in Sextans B. Whether this implies a general chemical inhomogeneity among populations of comparable age in Sextans B, and thus a metallicity spread at a given age, or whether we happen to see the short-lived effects of freshly ejected nucleosynthesis products prior to their dispersal and mixing with the ambient interstellar medium will require further study. For the PN we measured an O/H ratio of 12 + log(O/H) = 7.47 ± 0.16, consistent with that of the low-metallicity H II regions. We discuss the new metallicity data for the H II regions and PNe in the context of the published star formation histories and published abundances of the two dIrr galaxies. Both dIrrs show generally similar star formation histories in the sense of continuous star formation with amplitude variations but differ in their detailed enrichment timescales and star formation rates as a function of time. If we combine the photometrically derived estimates for the mean metallicity of the old red giant branch population in both dIrrs with the present-day metallicity of the H II regions, both dIrrs have experienced chemical enrichment by at least 0.8 dex (lower limit) throughout their history.